The present invention relates to a technology of inspecting and analyzing electronics, such as semiconductor devices.
A high-yield fabrication is demanded in manufacturing electronics, such as semiconductor devices like a semiconductor memory, typified by a dynamic random access memory, a microprocessor and a semiconductor laser, and a magnetic head.
Reduction in production yield caused by the occurrence of defective products lowers the profit. It is therefore an important issue to find defects, foreign matter and inadequate processing, which would bring about defective products, earlier and take an early countermeasure against such defects. For example, in the field of manufacturing electronics, attention is paid to detection of defective products by thorough inspection and analysis of causes for the defective products. In the actual electronics fabrication process using substrates, substrates after completion are inspected to investigate the locations of abnormality such as defects or foreign matter in a circuit pattern and consider a countermeasure against such defects.
Normally, a high resolution scanning electron microscope (hereinafter referred to as “SEM”) is used in observing the micro structures of samples. As the integration scale of semiconductor devices becomes greater, it becomes difficult to observe targets with the resolution of the SEM, so that a transmission electron microscope transmission electron microscope (hereinafter referred to as “TEM”) having a high observation resolution is used in place of the SEM.
Recently has been used a processing method which applies the action of particles constituting a sample to be discharged out of the sample when a focused ion beam (hereinafter referred to as “FIB”) onto the sample, i.e., FIB processing. Particularly, the use of FIB can ensure fabrication of a TEM sample without segmenting a wafer (see Japanese Patent Laid-Open Publication No. H5 (1993)-52721, for example). As shown in FIG. 2, the method carries out processes, such as the formation of a rectangular hole 101, a bottom hole 102, a trench 103, etc. by irradiation of an FIB 1, connection using an ion beam assist deposition layer 4 (hereinafter called “deposition layer 4”) and transfer of a micro sample 6 by means of a probe 3. As the micro sample is processed into a membrane by the FIB 1, it becomes a TEM sample. This scheme is called a micro-sampling method or pickup method.
A membrane 202 is formed on a wafer 201 as shown in FIG. 3A, and the periphery of the sample membrane is cut away, partly left, with the FIB 1 and a sample membrane 203 is separated from the wafer 201, as shown in FIG. 3B. Then, the wafer 201 is removed from the ion beam process system and the sample membrane 203 is completely separated from the wafer 201 in the air using the static electricity generated from a glass rod and is moved onto a TEM sample holder 204. This method can also ensure observation of a separated sample membrane with a TEM. This method is called a lift-out method.
There is also proposed a scheme of taking out a micro sample from a sample without segmenting a wafer using the sample segmentation method and returning the wafer to the next process (e.g., Japanese Patent Laid-Open Publication No. 2000-156393). This publication discloses a method which includes a step of extracting a part of a sample without segmenting a sample and preparing a TEM sample at the end of the fabrication process in the method of fabricating electronics by performing a plurality of processes on the sample, and monitoring or inspecting and analyzing the progress in the fabrication process. This method prevents semiconductor devices from being lost by the segmentation of a wafer and can thus reduce the total manufacturing cost of semiconductor devices.
Because the method uses an FIB containing gallium (Ga) as ions at the time of separating a micro sample, Ga constituting the FIB remains in the process area from which the micro sample has been removed. The presence of Ga is very likely to cause defective products in the fabrication of semiconductor devices. Particularly, as Ga is a p-type impurity with respect to an Si-semiconductor, the problem is more crucial. If a wafer is returned to the next process with contamination of Ga remaining, the pollutant Ga diffuses and permeates a semiconductor element which has undergone the fabrication process properly, causing improper electric characteristics or improper contact. As a solution to this problem, a wafer from which a micro sample is extracted may be subjected to cleaning using a chemical. This scheme however involves multiple steps, which would raise the manufacturing cost. Further, when the FIB is irradiated at the acceleration of 30 kv, for example, the FIB enters the sample to the depth of about 10 nm from the surface, so that surface cleaning alone cannot completely remove contamination containing an ion beam element buried in the sample.
A countermeasure against Ga contamination is proposed (see Japanese Patent Laid-Open Publication No. H6 (1994)-260129, for example). To return a sample irradiated with an FIB using Ga as an ion source to a process, the disclosed method removes a portion where Ga ions are implanted using an ion beam of a gas which does not seriously influence the characteristics of the sample or deposits an organic metal layer in such a way as to cover the Ga-implanted portion using a gaseous ion beam or an energy beam. That is, the publication discloses that after a process observation region is cleaned using one of argon (Ar), oxygen ion and oxygen radical and a compound is deposited there, the resultant sample is returned again to the fabrication process.
There is another proposition on the technology of processing a cross section with an Ar ion beam (see Japanese Patent Laid-Open Publication No. H7 (1995)-320670, for example). The publication discloses a scheme of processing a cross section for SEM observation using an Ar ion beam with a beam diameter of 0.1 μm generated from a helicon ion source. However, the publication discloses only a case where a target can be observed with the resolution of an SEM, but fails to give a consideration on TEM observation of a target which cannot be observed with the SEM.
The technique that does not generate defective products even by separating a sample for observation by a high-resolution electron microscope without segmenting a wafer and returning the wafer to the process without being contaminated with an element which would raise a problem in the process should still face the following problem.
First, the prior art techniques available to extract a TEM micro sample without segmenting a wafer are limited to schemes which use Ga ions, including the aforementioned micro-sampling method, pickup method and lift-out method. Therefore, studies are made on schemes which cope with Ga contamination. Of the conventional methods adapted to Ga contamination, the one which removes a Ga-implanted portion has to irradiate Ar ions on a wide region because Ga scatters widely, making the Ga contaminated region wider than the process region.
To completely eliminate Ga, Ar ions should be sputtered deeper than the process depth. This brings about problems that it takes time to cope with a process mark, made by the Ar ions, so that the mark does not raise a problem in the later process, and an extra cleaning step required after FIB processing would increase the manufacturing cost.
In the method of depositing an organic metal layer in such a way as to cover the Ga-implanted portion using a gaseous ion beam or an energy beam, the metal layer itself becomes a process contamination or the thickness of the metal layer makes the thickness of a sample at the peripheral portion different from the thickness of the other portion. This is likely to adversely affect the later process.
The conventional method of processing a cross section for SEM observation using an Ar ion beam with a beam diameter of 0.1 μm does not take, into consideration, TEM observation which cannot be observed with an SEM from the beginning. Like the conventional use of a GaFIB, the scheme forms a fine ion beam with a beam diameter of 0.1 μm and process a flat cross section perpendicular to the top surface. However, the luminance of the ion source that generates gaseous element ions, such as Ar, is lower by at least two digits to three digits than the luminance of a liquid metal ion source which is used to form a GaFIB. The formation of an ion beam with a beam diameter of 0.1 μm actually makes extremely difficult to achieve the desired level of 100 pA. If such a level is achieved, no way to keep the performance during the time practically needed has been achieved yet. Actually, the current obtained when the beam diameter is set to 0.1 μm merely has several pA so that the scheme has not made into a practical use.
That is, if the beam diameter of an Ai ion beam is made smaller to the size of a GaFIB, the current becomes small and the acceleration speed is too low, so that it is a common sense to those skilled in the art that an Ar ion beam is not usable. While wafer contamination by Ga ions is a well-known problem, there has been proposed no idea to prepare a TEM sample using Ar ions.